1. Field of the Invention
The present invention relates to a color cathode-ray tube of the shadow mask type and, more particularly, to a color cathode-ray tube capable of preventing images on the phosphor screen from being deteriorated by a thermal expansion of the shadow mask.
2. Description of the Related Art
Generally, a color cathode-ray tube comprises an envelope which includes a panel having a substantially rectangular effective surface formed of essentially a curved surface and a skirt portion provided at the periphery of the effective surface, and a funnel attached to the skirt portion of the panel. A phosphor screen comprising three-color phosphor layers which emit blue, green and red is formed on the inner side of the panel effective surface, and a substantially rectangular shadow mask is arranged inside and opposed to the phosphor screen. The shadow mask includes a mask body in the form of a curved surface and having a plurality of electron beam apertures in its area which is opposed to the phosphor screen, and a mask frame attached to the outer peripheral portion of the mask body.
An electron gun for emitting three electron beams is arranged in a neck of the funnel. Three electron beams emitted from the electron gun are deflected by magnetic field generated by a deflection yoke on the funnel and horizontally and vertically scan the phosphor screen through the shadow mask, thereby displaying a color image on the screen.
In order to display color images of good color purity on the phosphor screen, in the color cathode-ray tube constructed in this manner, the phosphor screen and the shadow mask must be arranged each other in a predetermined matching relation so that the three electron beams passing through the electron beam apertures of the shadow mask and entering into the phosphor screen correctly land on their corresponding three-color phosphor layers. To achieve this, it is important that, especially, the distance (or value q) between the inner face of the panel and the shadow mask is securely set as a designed value.
Even when the phosphor screen and the shadow mask are correctly arranged each other in the predetermined matching relation, however, the color cathode-ray tube deteriorates its color purity because of the thermal expansion of the shadow mask. Specifically, that area of the shadow mask in which the electron beam apertures are formed is smaller than 1/3 of the total area of the mask body. Most of electron beams, therefore, impinge against the shadow mask to thereby heat it. The mask body which is formed of a low carbon steel plate mainly including iron thus is heated to undergo thermal expand, and is subjected to doming such that it bulges toward the phosphor screen. As the result, the value q changes and the landing position of electron beams on the three-color phosphor layers also changes to thereby deteriorate color purity.
This change in the beam landing position (or mislanding) on the three-color phosphor layers caused by the thermal expansion of the shadow mask varies depending on image patterns on the phosphor screen and the time during which an image pattern is kept on the screen.
When images are displayed on the phosphor screen for a long time, the mask frame attached to the peripheral portion of the mask body and having a large heat capacity is also heated in addition to the mask body having a plurality of electron beam apertures, and they thermally expand together. The mislanding of electron beams caused by this thermal expansion can be effectively corrected by interposing bimetal elements between the mask frame and elastic supports for supporting the shadow mask, as disclosed in Jpn. Pat. Appln. KOKOKU Publication No. 44-3547.
If a high-luminance image is locally displayed for a relatively short period of time, the local mislanding of electron beams is caused, as a short time one. This local mislanding cannot be corrected by means of the bimetal elements. More specifically, when an image having a local high luminance is displayed on the phosphor screen by means of high current electron beams, the mask body 3 is subjected a local thermal expansion by the impingement of high current beams against it. In the thermally expanded portion of the shadow mask, each electron beam aperture is displaced from its normal position to an abnormal position. While the electron beams passing through the electron beam apertures which are positioned at the normal position correctly land on the three-color phosphor layers, those passing through the electron beam apertures which are positioned at the abnormal position cannot correctly land on the three-color phosphor layers. This mislanding of electron beams caused by the local thermal expansion of the mask body cannot be corrected by means of the bimetal elements because the thermal expansion is local.
The mislanding of electron beams caused in a short period of time was checked while changing the shape, size and position of a rectangular frame pattern generated by a signal generator. The mislanding of electron beams caused when a high current beam pattern is displayed substantially all over the phosphor screen is relatively small. When a high current beam pattern elongated in the vertical direction is displayed on the screen, however, it has been found that the mislanding of electron beams becomes largest in a case where the high current beam pattern is displayed on the portion of the phosphor screen which is slightly away from the horizontal end of the screen toward the center thereof.
The relationship between the high current beam pattern and the mislanding of electron beams can be described as follows.
A television set is usually designed in such a way that an average anode current applied to the cathode-ray tube should not exceed a given value. Therefore, when a large high-luminance beam pattern is formed on the phosphor screen, the beam current for each unit area of the shadow mask is lower, and the temperature rise of the mask is smaller, than when the small high-luminance beam pattern is formed. Further, when even the small high-luminance beam pattern is formed on the phosphor screen at the center portion thereof, the mislanding of electron beams cannot be easily caused even though the shadow mask is subjected to thermal expansion. As the originating position of the beam pattern shifts from the center of the phosphor screen toward the horizontal end portion thereof, the thermal expansion of the shadow mask appears, as the mislanding of electron beams, more frequently on the screen. However, near the peripheral portion of the phosphor screen, the shadow mask is attached to the mask frame, so that a deformation of the mask body caused by the thermal expansion is small. Accordingly, the mislanding of electron beams becomes largest not at the horizontal end portion of the phosphor screen but at the portion of the phosphor screen which is slightly away from the horizontal end toward the center of the screen.
Particularly in the FS (flat square) tube in which the effective area of the panel is made flat, the mask body is also made flat. The mislanding of electron beams is thus made more frequent by the thermal expansion of the shadow mask.
Disclosed in Jpn. Pat. Appln. KOKAI Publication Nos. 59-163737, 61-163539 and 61-88427 is means for restraining the mislanding of electron beams in the color cathode-ray tube, in which the effective area of the panel is flat, by changing the configuration of a flat shadow mask. However, the mislanding of electron beams cannot be fully corrected even if the configuration of the shadow mask is changed relative to the panel whose effective area is made flat.
Disclosed in Jpn. Pat. Appln. KOKAI Publication Nos. 64-17360 and 1-154443 is means for correcting the mislanding of the electron beams by changing the configuration of the effective area of the panel and that of the shadow mask. Even if this correction is made, however, a satisfactory effect cannot be obtained for a color cathode-ray tube having a substantially spherical flat panel which ensures a natural agreeable reflection on its outer surface and has recently stated to be used.
Further, the color cathode-ray tube whose panel has a flat effective surface involves the following problems, as well as the mislanding caused by the thermal expansion of the shadow mask.
In the color cathode-ray tube whose panel has a flat effective surface, the body of the shadow mask may be formed of a low-expansion material, such as Invar, besides a low-carbon steel sheet which is used for the shadow mask of a conventional color cathode-ray tube. Normally the shadow mask body is press-molded to have a predetermined curved surface after apertures are formed therein by photo-etching. In a conventional color cathode-ray tube whose mask body is formed of a curved surface with a relatively small radius of curvature, the mask body can be subjected to appropriate plastic deformation to obtain a necessary mechanical strength as it is press-molded. However, a flat shadow mask cannot be subjected to satisfactory plastic deformation and inevitably involves local low-strength portions, since the amount of deformation during the press-molding is small. Particularly in the case of the rectangular shadow mask, the central portions of the long and short sides of the mask body which are away from the corners of the mask, that is, the portions located near the ends of the horizontal and vertical axes of the mask body become low in mechanical strength. A countermeasure has been added to those portions of the mask body which are adjacent to the ends of the horizontal axis thereof, as disclosed in Jpn. Pat. Appln. KOKAI Publication No. 5-25885. However, those portions of the mask body which are adjacent to the ends of the vertical axis thereof are left unsolved and when impact and vibration are added to the shadow mask, therefore, those portion easily deform and resonant, causing a color drift.